FIG. 1 shows a schematic view of cells in a DECT system.
The portion of the radio system depicted comprises two fixed base stations 1 and 2 serving respective cells 3 and 4 representing areas of radio coverage which may be inside or outside buildings. The edges of further cells 7 and 8 are also shown. A user or subscriber to the system carries a portable handset 5 which is capable of two-way radio communication with one or other of the base states 1,2, and with other base stations. The system uses ten carrier frequencies, separated by 1.728 MHz, within a frequency band from 1880 MHz to 1900 MHz. This system divides time into TDMA frames, with each frame having a time duration of 10 ms. Each frame is divided into 24 time slots, numbered from 0 to 23. Each frame is divided into two halves, the first half (slots 0 to 11) being reserved for the transmission of the base station and the second half (slots 12 to 23) being reserved for the transmission of the handset. The handset carries the algorithm or algorithms for channel selection (i.e. selection of the combination of a particular time slot and carrier) in its processor.
Most currently manufactured DECT base stations contain a single transceiver and are therefore unable to open more than a single communication channel, a combination of time slot and frequency, on different frequency carriers in the same time slot. A time slot already having a traffic channel is `blind`. This means that although the number of channels available for selection is the number of carriers multiplied by the number of time slots, the maximum number of concurrent users is restricted to the number of available time slots i.e. twelve. The DECT standard includes a blind slot information message sent by a base station to inform the handset of time slots it should avoid for selecting a channel. In addition to slots `blinded` by the base station, a handset `blinds` channels itself as its frequency synthesiser is typically unable to switch to a channel at the next time slot but at a different frequency to that to which it is currently locked, i.e. the current channel.
The handset 5 monitors signals transmitted by a current base station 1 to receive system information via a current channel. A signal packet is transmitted each frame on active and dummy channels. Each signal packet is divided into four fields only one of which transmits system information. The signal packet structure is illustrated in FIG. 2. The S-field 10 used for synchronisation, and the A-field 11 used to send signalling information in accordance with the DECT protocol are both used when locking on. The B-field 1 2 is used for sending speech or data to a system user and the Z-field 13 is provided specifically for sliding error detection. In addition to the Z-field, the A and B fields have their own error detection sub-fields known as Cyclic Redundancy Checks 14 (CRCs). The A-field is 64 bits long with the final 16 bits providing an error check on the preceding 48 bits.
Information on the RSSI (Received Signal Strength Indicator) is measured, periodically, for the channels of the system, for example at a rate of once per two seconds. This is stored in the handset as a list. An example of such a list is shown in FIG. 3. Referring to FIG. 3, the twelve slots 1-11 (from the base stations) are shown as columns, with the ten carrier frequencies being represented by horizontal rows. Hence, the array of FIG. 3 has 120 boxes, each representing a particular channel, i.e. a particular combination of time slot and frequency. Signal strength is allocated a level from 0 to 14 (typically 6 dB bands are used), with 0 being the most quiet and 14 being the least quiet.
When a call is in progress, the handset 5 continuously measures the transmission error rate on the current channel using the CRC fields 14, and also continuously measures the RSSI information for the current channel. If the quality of the current channel deteriorates below an acceptable level as can be determined by the transmission error rate and/or the RSSI information for the current channel, the handset 5 can select an alternative channel and then attempt handoff to this channel. The handoff can either be to a different channel at the same base station (intracell) or to a different channel at a different base station (intercell). Once the quality of the current channel deteriorates below an acceptable level, the handset 5 has a finite number of attempts, currently 10 under the DECT standard, to handoff successfully before the system considers the call to be lost.
The reason for placing a limit on the number of attempts allowable to achieve successful handoff is to prevent handsets from contributing unnecessarily to the noise levels in the system by repeatedly and unsuccessfully trying to handoff to a particular channel. Anyway, after the time taken to make 10 handoff attempts has elapsed, the handset is losing synchronising with the system to such an extent that it is unlikely to be able to decode any system information signals.